Aliphatic amines are important organic intermediates which are produced on a large industrial scale. They are further processed for the production of agrochemicals or dyes for example, or they are used in surface-active formulations, as a corrosion inhibitor in lubricants or as auxiliaries in the paper, textile and rubber industries.
It is known to produce primary aliphatic amines from aldehydes and ammonia with hydrogen over a catalyst. This reaction is also known as a reductive amination. Amine formation can be described by the following reaction stages:R—C(═O)H+NH3→R—C(═NH)H+H2O  (1)R—C(═NH)H+H2→R—CH2—NH2  (2)
The first reaction stage eliminates water to produce an imine, which is subsequently subjected to a catalytic hydrogenation in a second reaction stage.
However, unwanted secondary reactions also occur. First, the feed aldehydes may become directly hydrogenated to the alcohol. Secondly, the feed aldehyde may undergo an aldol condensation in the basic medium and primary amine which has already been formed can react with the feed aldehyde via the azomethine intermediate to form a secondary amine which can then further react in a similar fashion to form a tertiary amine. Moreover, the aldol condensation products contain reactive groups which can combine with the nitrogenous compounds to form comparatively high-boiling condensation products. To improve selectivity in the direction of the primary aliphatic amines and to restrain the formation of high-boiling by-products, various measures have been proposed in the prior art, for example using excess ammonia or a solvent when it is likely that the reaction mixture will become inhomogeneous as a result of the water formed (Houben-Weyl, Methoden der organischen Chemie, 4th Edition, Georg Thieme Verlag Stuttgart, Volume XI/1, p. 602 ff.).
DE 936211 describes a liquid-phase process for producing primary aliphatic amines. In this process, the aldehyde to be reacted is first mixed with ammonia at temperatures below 0° C. Optionally, the aldehydes are diluted with a low-boiling alcohol, for example methanol. This mixture is then catalytically hydrogenated, for example over a cobalt or nickel catalyst, at elevated temperature and pressure in the upflow or downflow mode.
In DE 199 35 448 A1, a mixture of methanol and ammonia is admixed with Raney nickel and hydrogen and heated to reaction temperature. The aldehyde is then added. After the reaction has ended, the batch is depressurized to evaporate methanol and ammonia. The primary aliphatic amine left behind is then further reacted.
EP 0 628 535 A1 discloses first mixing the aldehyde with a diluent, for example methanol or water, at not more than 5° C. to suppress hemiacetal or hydrate formation. A separate reaction vessel is initially charged with ammonia, hydrogen and nickel catalyst at elevated temperature and pressure, liquid ammonia being present in the reaction vessel. The cooled mixture of aldehyde and diluent is introduced into the reaction vessel while stirring. After the reaction has ended, the reaction mixture is filtered through a frit and the crude mixture freed of the catalyst is worked up. The known process teaches the use of a high molar excess of ammonia. At least 15 and preferably from 20 to 50 mol of ammonia are used per mole of aliphatic aldehyde.
In the process of US 2008/0227632 A1 the reductive amination of alcohols, aldehydes or ketones is carried out in the presence of a catalyst which, in addition to nickel, copper and chromium, additionally contains tin as an active metal. The addition of tin proves to be advantageous for restraining the formation of hydrogenated by-products. The known process is particularly useful for aminating mono- or polyfunctional alcohols, for example ethylene glycol, diethylene glycol or triethylene glycol.
In addition to these single-stage processes, the prior art also features two-stage processes wherein the aliphatic aldehyde and excess ammonia initially combine in the presence of an imination catalyst to form the imine intermediate which is subsequently converted with hydrogen into the primary aliphatic amine by using a hydrogenation catalyst. In EP 0 816 323 A2, the imination catalyst used in this two-stage procedure is an organopolysiloxane comprising sulphonate groups. The subsequent hydrogenation is carried out in the presence of, for example, cobalt-, nickel- or ruthenium-containing catalysts.
Existing single-stage processes either utilize a solvent or diluent, or require pre-mixing of the feed components with cooling. The use of a solvent or diluent necessitates specific removal from the reaction mixture as well as additional logistical measures, such as recycling and storekeeping. Precions reactor capacity is occupied and plant throughput reduced. Pre-mixing the feed components with cooling and maintaining low temperatures in the mixture before entry into the reactor represents an additional technical inconvenience.
It is an object of the present invention to provide a process for producing primary aliphatic amines which is technically convenient and provides the desired primary aliphatic amines with high selectivity. More particularly, the formation of high-boiling by-products shall be restrained as far as possible.